Microtomes are employed for cutting samples, in particular biological tissue samples. For this purpose, a thin slice is drawn off from the tissue sample using a blade similar to a razor blade. The tissue sections thereby produced have a thickness which, depending on the sample consistency, lies in a range from micrometers to a few tenths of a micrometer. Thin tissue sections of this type may be observed and examined in a microscope in transmitted light. The layer thicknesses of the tissue sections must be dimensioned such that they have an adequate transmission of the illuminating light of the microscope.
To produce appropriately thin tissue sections, it is, possible to freeze the entire tissue sample or embed it in a substrate and harden it to form a solid body. Thin sections may be cut from solid samples relatively easily.
Often, however, thin sections must be produced from soft materials, such as are constituted by most living cell tissues and, in particular, brain tissue. In this case, the cell tissue is mostly located in an aqueous buffer solution. Following the severing of the tissue sample, the tissue section floats to the surface of the buffer solution. Soft materials cannot be sectioned particularly well by standard microtomes, since the material is uncontrollably misshapen during cutting. Therefore, microtomes having a vibrating blade are employed for such materials. In this case, the blade is set to vibrate parallel to the blade cutting edge. The blade cutting edge therefore vibrates transverse to the direction of advancing the cut, a method by which the cutting results are improved.
Vibration microtomes of this type are known from various manufacturers and in two different embodiments. The brochure "Vibratome Sectioning Products" from the firm of Ted Pella Inc., January 1992, 4595 Mountain Lakes; Boulevard, Redding, Calif. 96003, U.S.A., discloses a vibration microtome which uses an electromagnet to excite the oscillation of the blade. In this case, the blade holder is set oscillating by the electromagnet at a constant oscillation frequency which corresponds to the mains frequency of 50 Hz or 60 Hz. As a result of linear guidance of the blade holder, the oscillation of the blade takes place parallel to its cutting edge. The amplitude of the oscillation can be changed to a small extent by adjusting the coil current, in order to adapt somewhat to the sample material.
The brochures "EMS Oscillating Tissue Slicer" from the firm of Electron Microscopy Sciences, 321 Morris Road, Box 251, Fort Washington, Pa. 19034, and "Leica VT 1000 E/M High-Level Quality and Functionality" from the firm of Leica Instruments GmbH, P.O. Box 1120, 96226 Nussloch, Germany, disclose vibration microtomes in which the vibration movement runs parallel to the blade cutting edge, but their drive is carried out by means of an electric motor. The rotational movement of the electric motor is in this case converted into a linear movement via a push rod. Here, the oscillation amplitude remains constant, whereas the oscillation frequency can be adjusted via the rotational speed of the motor. Different oscillation frequencies are beneficial for different sample hardnesses.
DE-B 1 267 873 discloses a microtome having an oscillating blade, in which the oscillation frequency and amplitude are adjustable. For this purpose, two leaf springs are employed, each of which is fastened at one end to the microtome housing, and whose freely oscillating ends are connected to each other via a shaft bearing the microtome blade. The oscillation frequency is set by shortening or lengthening the effective spring length of the leaf springs by means of locking screws. The oscillation amplitude is changed by means of microphone armature structures, which act as electromagnets on the ends of the shaft. As a result of fastening the microtome blade via the shaft to the ends of the leaf springs, the microtome blade moves on a circular arcuate path.
Although the abovementioned vibration microtomes deliver better section results than standard microtomes having fixed blades, they are still not optimal. Furthermore, there is unevenness in the section surface of the sample, for which reason the sample has different layer thicknesses depending on the location. Wave-like layer thickness variations occur most often. Therefore, during an attempt to produce particularly thin sections, it often occurs that the tissue sections fall apart into individual strips, in accordance with the wave-like structures, and therefore become unsuitable for further use or examination. On the other hand, even in the case of cohering tissue, the wave-like thickness differences are disadvantageous and interfere with the examination in the optical microscope, since they bring about differences in brightness which are not founded in the material composition of the sample.